What Is Fast Startup in Windows? A Complete Guide (2026)

Windows boots faster than it used to, and Fast Startup is a big reason why. But most people have no idea it exists, what it actually does, or why it sometimes causes frustrating problems. This guide explains everything clearly, including when to keep it on and when to turn it off.

What Is Fast Startup in Windows?

Fast Startup is a Windows feature that makes your PC boot faster by saving a snapshot of the operating system kernel to your hard drive when you shut down, instead of doing a full shutdown. When you power the PC back on, Windows loads that saved state rather than initializing everything from scratch.

It was introduced in Windows 8 and has been present in Windows 10 and Windows 11. Microsoft quietly enables it by default on most systems.

The speed difference is noticeable. A cold boot on a traditional hard drive could take 45 to 90 seconds. With Fast Startup and an SSD, that same machine might be at the desktop in 10 to 15 seconds.

How Fast Startup Works Technically

To understand Fast Startup, you first need to understand two Windows states: hibernate and shutdown.

Normal Shutdown vs. Fast Startup Shutdown

When you do a traditional shutdown, Windows closes all running apps, logs out the user, and shuts down every system process including the kernel. On the next boot, everything loads from zero.

When Fast Startup is enabled and you click Shut Down, Windows does something different:

1. It logs out your user session (apps and files you had open are not saved).
2. It saves the state of the Windows kernel and system session to a file called hiberfil.sys on your C: drive.
3. It shuts the hardware down.

When you power on again, Windows reads that hiberfil.sys file and resumes the kernel state instead of booting fresh. Your user session still starts fresh, but the deeper system layer does not.

This is essentially a partial hibernate. The user side is clean, but the system core picks up where it left off.

What hiberfil.sys Is

hiberfil.sys is a hidden system file at the root of your C: drive. Its size varies but is typically a significant portion of your RAM. On a system with 16 GB of RAM, it can be 8 to 12 GB. This file exists whenever hibernation or Fast Startup is active.

Fast Startup vs. Sleep vs. Hibernate: Key Differences

People confuse these three constantly. Here is a clean breakdown.

Mode	What Gets Saved	Power Used	Boot Experience
Sleep	RAM stays powered (no file save)	Low (RAM stays live)	Resumes in seconds
Hibernate	Full RAM contents saved to disk	Zero (full power off)	Slower than sleep, faster than cold boot
Fast Startup	Kernel session only saved to disk	Zero (full power off)	Fastest boot from powered-off state
Normal Shutdown	Nothing saved	Zero	Slowest, fully fresh boot

The key takeaway: Fast Startup is not the same as Sleep. Your PC is fully powered off. It just has a head start on the next boot.

Restart bypasses Fast Startup entirely. When you click Restart in Windows, it performs a full shutdown and a full fresh boot. This is why IT professionals often tell you to Restart, not Shut Down, when troubleshooting.

Why Fast Startup Causes Problems

Fast Startup sounds great on paper. In practice, it creates specific, well-known issues that are worth understanding.

1. Windows Updates May Not Apply Properly

Some updates require a full shutdown and clean boot to complete. If you always use Shut Down with Fast Startup enabled, the kernel state is preserved from before the update. The update may partially apply or appear incomplete until you do a proper Restart.

2. Dual Boot Systems Break

If you dual-boot Windows alongside Linux or another operating system, Fast Startup is a serious problem. When Windows saves its kernel state to hiberfil.sys, it keeps the NTFS partition locked. When Linux tries to mount that same partition, it sees it as “in use” and either mounts it read-only or refuses to mount it at all. Your files on the Windows partition become inaccessible or risk corruption.

This is one of the most common causes of Linux-Windows dual boot filesystem issues.

3. Hardware and Driver Changes Do Not Register Cleanly

If you plug in new hardware or update a driver and then use Shut Down, the kernel that loads next time may not pick up those changes cleanly. Again, a Restart solves this because it does a full fresh boot.

4. BIOS and UEFI Settings May Be Inaccessible

Some systems make it harder to enter BIOS/UEFI settings when Fast Startup is on because the POST sequence is partially skipped. If you are trying to press F2 or Delete to get into BIOS on boot, it may not respond as expected.

5. Shared Drives and Encryption

If you use BitLocker or share drives between operating systems, the locked partition state from Fast Startup can interfere. Encrypted volumes may not hand off cleanly.

How to Check If Fast Startup Is Enabled

Here is how to check your current setting.

1. Press Windows + R, type powercfg.cpl, and press Enter.
2. In the Power Options window, click Choose what the power buttons do in the left panel.
3. Click Change settings that are currently unavailable (this unlocks the shutdown settings).
4. Look under Shutdown settings for the checkbox labeled Turn on fast startup (recommended).

If it is checked, Fast Startup is on. If the option is grayed out, hibernation is disabled on your system, which also disables Fast Startup.

How to Enable or Disable Fast Startup in Windows

Method 1: Through Power Options (Recommended)

Follow the same steps above. Simply check or uncheck the Turn on fast startup box, then click Save changes.

Method 2: Using Command Prompt

Open Command Prompt as Administrator.

To disable Fast Startup:

powercfg /hibernate off

To enable it again:

powercfg /hibernate on

Note that turning hibernation off entirely will also remove the hiberfil.sys file and free up that disk space.

Method 3: Group Policy (Windows Pro and Enterprise)

If you manage multiple machines or the setting is locked in your organization:

1. Press Windows + R, type gpedit.msc, press Enter.
2. Navigate to: Computer Configuration > Administrative Templates > System > Shutdown.
3. Find Require use of fast startup and set it to Disabled.

Should You Turn Off Fast Startup?

This depends on your setup. Here is a practical guide.

Keep Fast Startup ON if:

• You use Windows as your only operating system.
• You are on a traditional hard drive and every second of boot time matters.
• You never have issues with updates or hardware not registering.
• You are using a laptop and want slightly faster resume times.

Turn Fast Startup OFF if:

• You dual-boot Linux or any other OS alongside Windows.
• You work with external drives that sometimes do not show up after reboot.
• You are troubleshooting persistent software or driver issues.
• You use BitLocker or full-disk encryption in a shared environment.
• Your Windows updates seem to stall or not complete properly.
• You have an NVMe SSD and your machine already boots in under 10 seconds anyway (at that point, the benefit is minimal).

For most home users on a single-boot Windows 11 system with an SSD, keeping Fast Startup on is fine. For power users, developers, or anyone with a complex setup, turning it off removes a common source of confusing problems.

Does Fast Startup Affect Performance Beyond Boot Time?

Fast Startup only affects the boot sequence. Once Windows is running, it has no effect on performance. Apps run the same, RAM usage is the same, and nothing is “carried over” from the previous session except the kernel state, which is immediately overwritten as the system runs.

There is a small theoretical argument that a stale kernel state could contribute to memory issues over very long uptime, but in practice this is not a meaningful concern for normal use.

Fast Startup and SSD Lifespan

Some users worry that Fast Startup writes more data to the SSD (because of the hiberfil.sys save on every shutdown), which could accelerate drive wear. This concern is technically valid but practically insignificant. Modern SSDs have write endurance measured in terabytes of data. The hiberfil.sys write on each shutdown adds a few gigabytes per week at most, which is well within normal SSD wear parameters for many years of use.

If you are on an older SSD with very limited write endurance or a low-endurance budget drive, you could make a case for disabling it. Otherwise, it is not worth worrying about. You can read more about SSD endurance and TBW ratings at Crucial’s SSD endurance guide.

Fast Startup and Windows 11

Windows 11 behaves the same as Windows 10 regarding Fast Startup. The feature is enabled by default and lives in the same Power Options menu. One difference worth noting: Windows 11 with modern UEFI and NVMe SSDs boots so fast that many users report they cannot notice the difference between Fast Startup on and off. The feature matters more on older hardware.

Microsoft’s official documentation on power and sleep settings, including Fast Startup, can be found at Microsoft’s Power and Sleep FAQ.

Common Questions About Fast Startup

Does Fast Startup affect RAM usage?

No. RAM is cleared when the PC powers off. Fast Startup saves the kernel to disk, not RAM. When the PC turns on, that kernel state is loaded back from disk into RAM fresh.

Why does my external drive sometimes not appear after shutdown?

Fast Startup can preserve USB and storage controller states in ways that confuse the system on next boot. Disabling Fast Startup often fixes this. Alternatively, doing a Restart instead of Shut Down also resolves it since Restart bypasses Fast Startup.

Can I tell if my last boot used Fast Startup?

Yes. Open Event Viewer, go to Windows Logs > System, and look for Event ID 1. Fast Startup boots show as Resume events rather than standard boot events.

Does Fast Startup save battery on laptops?

No. The machine is fully powered off. There is no battery draw. The only benefit is faster next boot.

Summary

Fast Startup is a Windows feature that speeds up boot time by saving the Windows kernel state to disk on shutdown instead of closing everything cleanly. It is enabled by default and works well for most single-boot Windows users.

Here is the short version of everything covered:

• Fast Startup uses a partial hibernate called hiberfil.sys to save the kernel session.
• Restart always bypasses Fast Startup and does a full fresh boot.
• It can break dual-boot setups, prevent updates from applying correctly, and cause external drive issues.
• Disabling it is straightforward through Power Options or a single command.
• On modern SSDs, the performance difference is often unnoticeable.

If your machine is running Windows on its own without any dual-boot configuration and you are not chasing troubleshooting issues, leave Fast Startup on. If anything feels off after updates or your dual-boot Linux setup is acting strange, disabling it should be one of the first things you try.